One night of disturbed sleep increases levels of a brain protein linked to Alzheimer's disease, research has shown.

A week of tossing and turning boosts another molecule implicated in the destruction of brain cells, the same study found.

Increasing evidence suggests an association between poor sleep and a greater risk of cognitive impairment or dementia.

The new findings published in the journal Brain show that just one bad night can affect a key signature of Alzheimer's in the brain.

Lead scientist Professor David Holtzman, from Washington University School of Medicine in St Louis, US, said: "We showed that poor sleep is associated with higher levels of two Alzheimer's-associated proteins.

"We think that perhaps chronic poor sleep during middle age may increase the risk of Alzheimer's later in life."

Brains of Alzheimer's patients are clogged with accumulated deposits of a toxic protein called beta-amyloid. In addition, their neurons contain "tau tangles" - twisted knots of tau protein that disrupt nutrient transport within the cells and eventually kill them.

Experts believe both abnormalities contribute to Alzheimer's disease and may be linked. One theory suggests that beta-amyloid acts as the "trigger" and tau as the "bullet" in a process that leads to the death of neurons.

Prof Holtzman's team looked at 17 healthy adults aged 35 to 65 with no history of chronic sleep problems or mental impairments.

The volunteers agreed to spend a night in a specially designed, sound-proofed sleep room, while wearing headphones as their brain waves were monitored via electrodes attached to the scalp.

Half the participants were randomly assigned to have their slumber disrupted by beeps in their ears once their brain activity showed them to be in a deep dreamless phase of "slow-wave" sleep.

About a month later the process was repeated, but this time the two groups were reversed so that participants who previously had an undisturbed night were subjected to the beeps.

Tests on cerebrospinal fluid taken from the volunteers showed a 10% increase in beta-amyloid levels after a single night of interrupted sleep, but no increase in levels of tau.

However, participants who had slept poorly at home for a week before the spinal tap did display a spike in tau levels.

Co-author Dr Yo-El Ju, also from Washington University, said: "We were not surprised to find that tau levels didn't budge after just one night of disrupted sleep while amyloid levels did, because amyloid levels normally change more quickly than tau levels.

"But we could see, when the participants had several bad nights in a row at home, that their tau levels had risen."

Slow-wave sleep is a time when neurons rest and the brain clears away molecular by-products of mental activity that accumulate during the day.

Dr Ju thought it unlikely that even a week of poor sleep would affect the overall risk of developing Alzheimer's disease. A return to sleeping normally probably caused beta-amyloid and tau levels to return to normal, she said.

"The main concern is people who have chronic sleep problems," Dr Ju added.

"I think that may lead to chronically elevated amyloid levels, which animal studies have shown lead to increased risk of amyloid plaques and Alzheimer's.

"At this point, we can't say whether improving sleep will reduce your risk of developing Alzheimer's. All we can really say is that bad sleep increases levels of some proteins that are associated with Alzheimer's disease.

"But a good night's sleep is something you want to be striving for anyway."

Dr Laura Phipps, from the charity Alzheimer's Research UK, said: "There is mounting evidence of a link between poor quality sleep and Alzheimer's disease, but it is difficult to tease apart cause and effect in this relationship and determine whether sleep problems might cause Alzheimer's brain changes or vice-versa.

"This very small study in younger people suggests lower quality sleep can trigger changes in the levels of proteins linked with Alzheimer's, strengthening suggestions that sleep is important for the turnover of these proteins in the body.

"While this study sheds more light on the link between sleep and Alzheimer's proteins, it doesn't tell us whether this short-term association is relevant for the development of Alzheimer's disease long-term. The development of Alzheimer's is a process that takes many years and is likely to depend on multiple genetic, health and lifestyle factors.

"There are a number of important health benefits linked to a good night's sleep but further research is needed to unpick the potential long-term benefits of sleep on Alzheimer's risk."